1. Field of the Invention
The present invention relates to a screw (bolt) and a stud hole with a looseness resisting function, which are applied to various types of electronic instruments such as a cell phone, a digital camera, a liquid-crystal or plasma television, and a personal computer, as well as office equipment including a copy machine and a printer. The screw requires an adequate screwing torque to be screwed with ease and has an appropriate securing force to surely prevent loosening thereof.
2. Background Art
Recently, an extremely thin metal sheet or a low-friction plastic material is used for a chassis or a housing to realize lightweight properties or compact designs in various types of electronic instruments such as a cell phone, a digital camera, a liquid-crystal or plasma television, and a personal computer, as well as office equipment including a copy machine.
Particularly, an instrument having a high-voltage heat source such as a condenser like a liquid-crystal or plasma television, a copy machine, and a printer requires a heat transfer passage. To allow such a heat transfer passage, compact screws and stud nuts (spacers) are to be provided for assembling such an instrument.
In such an industrial field, a looseness resisting screw is needed to secure elements on a thin metal plate of about 0.5 mm or on a low-friction plastic member.
A conventional looseness resisting screw engages at a frank (opposing) angle of about 1° 20′ to 3°30′ with an associated internal thread, and top and root parts of the screw and the internal thread contact each other.
The conventional looseness resisting screw is screwed into the internal thread so that the thread top part of the screw is pressed against the internal thread. This prevents loosening of the screw. Such a screw is disclosed in Japanese Patent Application Laid-open NO. S-61-286605.
However, the conventional looseness resisting screw and the associated internal thread each have a uniform thread section with a predetermined dimensional allowance. Therefore, a fastening friction force increases with advancement of the screw into the internal thread, since the top part of the screw thread is pressed into the internal thread. This increases a securing torque of the screw, causing a difficult fastening and securing operation of the screw.
Furthermore, the conventional looseness resisting screw and the associated internal thread produce a larger amount of cut-off chips since all the top parts of the screw thread are pressed into the internal thread. This prevents a smooth and quick securing operation of the screw into the internal thread.
Furthermore, the conventional looseness resisting screw and the associated internal thread have a predetermined dimensional difference between thread profiles of the screw and the internal thread. To improve a looseness resisting function of the screw and the internal thread, the dimensional difference needs to be improved.